Regulation of transcription was studied in a wild-type transposable phage (TP) D3112 of Pseudomonas aeruginosaand its mutants for different genes. For this purpose, [3H]RNA–DNA hybridization with denatured fragments from various regions of the D3112 genome bound to nitrocellulose filters was used. A transcriptional map of TP D3112 was constructed based on the data obtained. The map comprised six independent transcriptional units corresponding to the modular organization of the phage genome. Only the repressor gene cI was transcribed in the lysogenic state. After repressor thermoinactivation, the cI transcription ceased, and transcription occurred in the same order as the genes (modules) were located on the D3112 phage genetic map (from left to right): cip1(a negative regulator) and early genes A and B (controlling the replication–transposition of the phage), nonessential genome region, genes C and the locus marked with the ts47 mutation (positive regulators of late gene transcription), the genes of the head morphogenesis, gene c91(a positive regulator responsible for the lysogenic state), and the genes of the tail morphogenesis. Similarities between nonhomologous TPs D3112 of P. aeruginosa and Mu of Escherichia coli with respect to genetic organization and transcription regulation are discussed.